1. Field of the Invention
This invention relates to a paint composition dispersible in water and which can be suspended in water or in a medium mainly comprising water to yield a highly stable suspension suitable as a water-base paint.
2. Description of the Prior Art
In general, the methods of application of synthetic resin powder as a paint or coating can be divided roughly into two groups. One group comprises application of the powder by itself as in the fluidized bed method or the electrostatic coating method. The other group comprises application of a suspension of the synthetic resin powder in a dispersion medium. Generally, the latter method is preferred to the former in most cases, because the former presents difficulties in workability and in controlling the coating thickness owing to scattering of the powder dust. However, in the latter method, an organic solvent is generally used at present as the dispersion medium, and this is undesirable from the viewpoints of workability and human health.
On the other hand, if a process for dispersing the synthetic resin powder in a medium mainly comprising water is employed, the paint thus obtained can be applied onto the surface of the object to be coated, such as a metal object. The coated object can then be heated to evaporate the solvent mainly comprising water and then further heated to a temperature above the melting point of the synthetic resin to melt the resin, thereby forming on the object a coating film. Thus, a coating film of any desired thickness having an excellent adhesive property and smoothness can be obtained without damaging the essential properties of the synthetic resin as is apt to happen with an organic solvent. Therefore, the disadvantages caused by the use of an organic solvent can be overcome.
But, in fact, it is very difficult to suspend, in a medium mainly comprising water, a resin powder having an average particle size of about 0.1 to 250 .mu. prepared from masses or grains of a synthetic resin by a mechanical pulverization or precipitation of the resin powder from a solvent solution of the resin. Such a powder cannot be dispersed readily, by itself, in the medium, because its particle size is larger than that of the so-called emulsion obtained by carrying out the polymerization per se in an aqueous medium.
When a water-soluble polymer such as polyvinyl alcohol or carboxymethylcellulose (CMC) is used as a dispersing agent, the resultant coated film has a poor thermal resistance and a severely reduced adhesive strength, and furthermore, the film is heavily colored.